During the harvesting of forage material it is common practice to drive a truck or wagon alongside of the harvester for the purpose of periodically filling the truck with materials that have been accumulated by the harvester during the truck's absence to discharge its load. Desirably, the harvesting process is carried out continuously, but, it is not at all uncommon for the harvester to temporarily stop further operations while awaiting the return of an empty truck or trailer in view of the limited holding capacity of the harvester. In this respect, governmental height and width restrictions with respect to vehicles which at least periodically utilize public roads place constraints on the holding capacity of container boxes used by such harvesters, and, moreover, the shapes and dimensions of the boxes have effects upon their loading and unloading characteristics. In this regard, inclined or converging side boxes normally fill more completely and evenly than upright sided boxes, for example, and the point of discharge from the boxes must be sufficiently elevated to safely clear the truck or trailer into which the contents are unloaded.